This invention relates to fuel control systems for gas turbines. In particular, this invention relates to fuel trimming systems for industrial gas turbines having a plurality of combustion chambers.
Industrial gas turbines are required to perform at higher and higher efficiencies while producing less and less undesirable air polluting emissions. Higher efficiencies in gas turbines are generally achieved by increasing overall gas temperature in the combustion chambers of the gas turbine. Emissions are reduced by lowering the maximum gas temperature in the combustion chamber. The demand for higher efficiencies which results in hotter combustion chambers conflicts to an extent with the regulatory requirements for low emission gas turbines.
The primary air polluting emissions produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen (NOx), carbon monoxide (CO) and unburned hydrocarbons (UHC). The oxidation of molecular nitrogen in gas turbines increases dramatically with the maximum hot gas temperature in the combustion reaction zone of each combustion chamber. The rate of chemical reactions forming oxides of nitrogen is an exponential function of temperature. The volume of NOx emissions can be very great even if the hot maximum temperature is reached only briefly. A common method for reducing NOx emissions is to lower the maximum hot gas temperature in the combustion chamber by maintaining a lean fuel-air ratio.
If the fuel-air mixture in a combustion chamber is too lean, then excessive emissions of carbon monoxide and unburned hydrocarbon occur. CO and UHC emissions result from incomplete fuel combustion. Generation of these emissions usually occurs where the fuel-air mixture excessively quenches combustion in the reaction zone. The temperature in the reaction zone must be adequate to support complete combustion or the chemical combustion reactions will be quenched before achieving equilibrium. Unfortunately, prematurely quenched combustion too often occurs in current low-NOx combustors that operate with fuel-air mixtures near the lean limit of flammability.
The rates of CO and UHC emission generation due to combustion quenching are non-linear functions of reaction zone temperature and peak sharply at the lean fuel-air ratio limit of flammability. To minimize CO and UHC emissions, the reaction zones of gas turbine combustors should have adequate fuel-air mixtures to avoid the lean limit of flammability. However, combustors must still operate with lean fuel-air mixtures to reduce NOx emissions. To balance the conflicting needs for reduced CO, UHC and NOx emissions, extremely precise control is required over the fuel-air mixture in the reaction zones of the combustors in an industrial gas turbine.
The fuel-air ratio in each combustion chamber of a gas turbine should be the same. A constant fuel-air mixture in each combustor allows the mixture to be maintained at the lean ratio that best reduces CO, UHC and NOx emissions. In addition, uniform fuel-air ratios among chambers ensures a uniform distribution of temperature among the combustors of a gas turbine. A uniform distribution of temperature and pressure reduces the thermal and mechanical stresses on the combustion, turbine and other hot stream components of the gas turbine. A reduction in these stresses prolongs the operational lives of combustor and turbine parts. Peak hot gas temperature in some combustion chambers (but not others) increases thermal stresses and reduces the strength of materials in the hotter high fuel-air ratio chambers and turbine parts immediately downstream of those chambers.
It has proven extraordinarily difficult to achieve truly uniform temperature and pressure distribution in multiple combustion chambers of industrial gas turbines. For example, the air flow distribution in combustion chambers is perturbed by variations in the components of the combustion chambers and their assembly. These variations are due to necessary tolerances in manufacturing, installation and assembly of the combustor and gas turbine parts. In addition, the air flow paths arc irregular approaching the combustion system from the compressor and exiting at the combustor discharge to the turbine. These irregular paths affect the air flow through the combustor and cause a non-uniform air flow distribution in the combustors. For example, localized air flow resistance is caused by the lines for turbine bearing lube oil in the compressor discharge air flow path. The irregular air flow distribution among combustion chambers affects the fuel-air ratio differently in each combustion chamber. Variations in the air flow in each combustion chamber make it difficult to maintain constant fuel-air ratios in all combustion chambers.
Prior fuel systems for multiple combustion chamber industrial gas turbines provide uniform fuel flow distribution among the chambers. These systems have a common control that meters the same rate of fuel to each chamber. These systems do not trim the fuel flow to each combustion chamber to maintain a uniform fuel-air ratio in each chamber. Accordingly, these prior fuel systems cannot maintain a truly uniform fuel-air ratio in all combustion chambers when the air flow is not uniformly distributed among combustion chambers.
One proposed system for overcoming the above-described problems involves trimming the fuel flow to each combustion chamber in a multiple chamber gas turbine combustion system, as described in U.S. Pat. No. 5,661,969 issued to Beebe et al, commonly assigned to the General Electric Co., and incorporated herein by reference. The fuel flow distribution among chambers is trimmed to match the air flow distribution to obtain a uniform distribution of fuel-air ratios among chambers. Optimal fuel trimming equalizes the fuel-air ratio in all chambers regardless of uncontrolled chamber-to-chamber variations in the air flow.
The control signals to the fuel trimming system are: (1) individual combustion chamber fuel flow rates, (2) individual combustion chamber dynamic pressure levels, and (3) gas turbine exhaust temperature distribution around the entire turbine discharge. These signals may be used individually or in combination to determine the optimum fuel flow distribution set by the trimming system. Conventional instrumentation for each combustion chamber is used to obtain these control signals for the fuel trimming system. These instruments are well known in the gas turbine industry and have proven reliable.
One embodiment of the system described in Beebe et al is a gas turbine comprising a compressor, a multi-chamber combustor receiving pressurized air from the compressor, a turbine drivingly connected to the compressor and receiving exhaust from the combustor, a fuel system for providing fuel to each chamber of the multi-chamber combustor, where the fuel system trims the fuel to individual chambers to match the air flow to each chamber.
Similarly, another embodiment of Beebe et al involves a combustion section of a gas turbine having a plurality of chambers, at least one of the chambers comprising: at least one combustion reaction zone receiving air from a compressor and fuel from a fuel distributor; the fuel distributor having a fuel trim orifice and a fuel trim valve, the fuel trim valve for the at least one chamber being individually set to trim the flow of fuel to the chamber.
The advantages provided by these systems described in Beebe et al include uniform distribution of fuel-air ratios among multiple combustion chambers to minimize the emissions of objectionable air pollutants in the gas turbine exhaust, including nitrogen oxide, carbon monoxide, and unburned hydrocarbons over the entire load range of a gas turbine. In addition, uniform distribution of fuel-air ratio prolongs the operational life of the hot stream components of the gas turbine.
Thus, the systems described in Beebe et al provide a method for obtaining a uniform distribution of fuel-air ratio among all the combustion chambers of a multiple chamber combustion system in an industrial gas turbine. In particular, the described systems maintain a uniform fuel-air ratio in each chamber of a multiple chamber gas turbine combustion chamber system, when air flow is not uniformly distributed among the combustion chambers. The systems operate to trim the fuel flow distribution among the combustion chambers to match variations in air flow to each chamber.
The current invention features linked feedback control of those machine operating parameters that are critical to achieving maximum performance while meeting emissions requirements and optimizing hardware life. Specifically the invention calls for an array of sensors and fuel flow control to individual combustion chambers to provide input and control methods to a constrained optimization control algorithm (such as the constrained gradient-search technique) that will operate the turbine in a manner to achieve optimum levels of the critical parameters. The current invention directly results in improved control of the fuel to air ratio and dynamic pressures in each individual combustion chamber to optimize their individual performances.
The present invention utilizes feedback control to optimize performancexe2x80x94output and efficiencyxe2x80x94and emissions by deploying sensors in, and controlling fuel flow to, individual combustor cans in a multichamber machine. The machine may have either a conventional diffusion combustor with diluent such as water injected into the combustors or a lean premixed Dry Low NOx (DLN) combustion system.
The invention minimizes the emissions, notably nitrogen oxide, that are generated because of the inherent chamber-to-chamber variability that exists in a multi-chamber machine and results in the need to apply xe2x80x9cworst casexe2x80x9d control means for dynamics, etc. This results in being able to improve output and efficiency from a given gas turbine (by increases in, e.g., firing temperature and pressure ratio) while still meeting emissions.
The invention maximizes the power output capability of the gas turbine while maintaining the total emissions and dynamic pressure oscillations in each combustion chamber within acceptable limits. This invention performs this optimization in the presence of varying load conditions and varying ambient conditions.